1. Technical Field
This invention relates to the technology of Purifying exhaust gases or emissions of stationary and mobile sources, and more particularly to the technology of cleansing such emissions of NO.sub.x from an oxygen-rich stream.
2. Discussion of Related Art
Emissions from large scale stationary sources, containing NO.sub.x and excess O.sub.2, are generally treated with ammonia as a reductant over a catalyst containing V.sub.2 O.sub.5 on TiO.sub.2 (see H. Bosch and F. Janssen, "Catalysis Today", Vol. 2 (4), 1987). Emissions from mobile sources that do not have excess oxygen but contain NO.sub.x (automotive vehicles) are removed by reaction with in-situ reductants, such as carbon monoxide or hydrocarbons (HC), when passed over a catalyst, often containing rhodium Such a catalyst would be ineffective in the presence of a large excess of O.sub.2 (see K.C. Taylor, "Automobile Catalyst Converters", Springer, Berlin, 1984).
Recently, copper-exchanged zeolites have been found to reduce NO.sub.x in the presence of excess oxygen (see U.S. Pat. No. 4,934,142 and Japanese patent application publication No. Hei 3-52644, 3/6/91), but to attain substantial conversion efficiencies at the moderate temperatures of an exhaust produced by a lean-burn engine, a temporary rich A/F condition is required to provide a residual HC reductant. Unfortunately, it is not desirable to operate an automotive engine or other emission source under artificially rich conditions simply to facilitate catalytic conversion of the engine emissions (see M. Iwamoto et al, Applied Catalysis, Vol. 69, L 15-19, 1991). To facilitate conversion of NO.sub.x under conditions of excess oxygen, without the need for stimulating high HC in the exhaust, alkane and alkene additions have been suggested as reductants injectable into the emissions ahead of the zeolite (see S. Sato et al, Applied Catalysis, Vol. 70, L 1-5 (1991)). However, it is difficult to meter small doses of such gaseous reductants to match accurately the varying NO.sub.x content of the exhaust gas and to reliably and safely store such gaseous reductants on-board a vehicle.
It is therefore an object of this invention to provide a catalyst system for selective and efficient reduction of NO.sub.x accompanied by excess oxygen by use of a liquid reductant that provides several advantages not attainable by the prior art: (i) the reductant is easy to meter and match to the NO.sub.x variability, (ii) the reductant is nontoxic and safe to store for periodic use, and (iii) the reductant provides substantial enhancement of NO.sub.x conversion over a copper-exchanged zeolite at small excess oxygen conditions.